The present invention relates to a multiple output type stabilizing power source apparatus for generating stabilized AC and DC voltages.
A typical conventional multiple output type stabilizing power source apparatus as shown in FIG. 5 supplies a stabilized DC voltage to a DC load such as electronic equipment and a stabilized AC voltage to an AC load such as a fluorescent lamp.
Referring to FIG. 5, the stabilizing power source apparatus has inverters at the primary and secondary winding sides. A reference symbol V.sub.0 denotes a DC voltage; Q.sub.1, a switching transistor; and T.sub.1, an inverter transformer. The inverter transformer T.sub.1 has a primary winding N.sub.P and a secondary winding N.sub.S Reference symbols D.sub.1 and D.sub.2 denote rectifying diodes, respectively; L.sub.1, a choke coil; and C.sub.1, a smoothing capacitor.
A reference symbol A.sub.1 denotes a differential amplifier for comparing a reference voltage V.sub.R with a DC output V.sub.1 from the secondary winding N.sub.S and generating a signal corresponding to the difference. A reference symbol OSC denotes an oscillator for generating a triangular wave signal; and PWM, a pulse width modulator for converting the triangular wave from the oscillator OSC to pulses of the same frequency as that of the triangular wave and for changing a pulse width of the converted pulses in accordance with a shift of a crossing point between the triangular wave from the oscillator OSC and the signal from the differential amplifier A.sub.1. A reference symbol DR denotes a driver for amplifying the pulses from the pulse width modulator PWM and for supplying an amplified signal to the base of the transistor Q.sub.1.
A reference symbol INV denotes an inverter for receiving the DC voltage V.sub.1 from the secondary winding N.sub.S and for generating an AC voltage upon switching; T.sub.2, a voltage transformer for transforming the AC voltage from the inverter INV; C.sub.F, a ballasting capacitor; and R.sub.L, an AC load for receiving an AC output V.sub.2.
It should be noted that a resistor R.sub.S detects a current flowing through the AC load R.sub.L, and the inverter INV is switched to keep the current constant in response to the detection signal.
In the conventional stabilizing power source apparatus having the structure described above, a rectangular wave current obtained upon switching of the transistor Q.sub.1 is rectified by the diodes D.sub.1 and D.sub.2 connected to the secondary winding N.sub.S. The rectified current is smoothed by the choke coil L.sub.1 and the capacitor C.sub.1 to provide the DC output V.sub.1.
The DC output V.sub.1 is supplied to the differential amplifier A.sub.1 and is compared with the reference input voltage V.sub.R. The differential amplifier A.sub.1 provides a signal corresponding to the difference to the pulse width modulator PWM.
In the pulse width modulator PWM, the triangular wave from the oscillator OSC is converted to the pulses of the same frequency as that of the triangular wave. In this case, the pulse width is varied in accordance with the crossing point between the leading ramp of the triangular wave and the output from the differential amplifier A.sub.1. The pulse is amplified by the driver DR, and the amplified pulse is supplied to the base of the transistor Q.sub.1. The duty factor of the transistor Q.sub.1 is thus controlled in response to the output from the differential amplifier A.sub.1. Therefore, the DC voltage V.sub.1 is kept constant.
On the other hand, the stabilized DC output V.sub.1 is supplied to the inverter INV and is converted to the AC voltage upon its switching. The AC voltage is boosted by the transformer T.sub.2. The boosted voltage is then applied to the AC load R.sub.L through the capacitor C.sub.F. The function of the capacitor is as follows. Then R.sub.L is a discharge lamp, R.sub.L shows a negative resistance characteristic due to an arc in the lamp. To stabilize lamp current, it is generally required that a ballasting impedance is connected in series with the lamp. In this circuit, the capacitor C.sub.F provides the impedance.
When the AC load R.sub.L such as a discharge lamp is driven by the stabilizing power source apparatus described above, an AC voltage which is 3 to 5 times that in normal operation is required to start discharge. Since the capacitor C.sub.F is inserted at the output side of the transformer T.sub.2 and the output voltage at the transformer T.sub.2 drops to provide the normal operation voltage for the load R.sub.L, no voltage drop occurs across the capacitor C.sub.F at the start of discharge when no current flows through the load R.sub.L. Thus, a high AC voltage is applied to the load R.sub.L.
In the conventional stabilizing power source apparatus, however, since the switching circuits are arranged at the primary and secondary sides of the transformer, the circuit arrangement is complicated.
Furthermore, since the rectangular wave voltage is supplied to the primary winding N.sub.P of the inverter transformer T.sub.1, an AC ripple is generated by capacitive couplings between the primary and secondary windings N.sub.P and N.sub.S. This ripple has the same phase as that of the normal signal component, so that the ripple is fed back to the primary winding N.sub.P through the ground line at the secondary winding N.sub.S. Therefore, common mode noise including a comparatively high frequency component is generated.
In the conventional apparatus of this type, a large line filter must be inserted at the input side to reduce such noise.